Re: Non-PK encryption not vulnerable via low key length?!

[softtest () wu1 wl aecl ca (Software Test Account)]

On Thu, 16 Mar 1995, That Whispering Wolf... wrote:

> > > Correct me if I am wrong - RC2 and RC4 are not public key cyrptosystems,
> > > and hence are not "prone" to the problems with low moduli.
> >
> >    You are wrong.
> >
> >    If the key is only 128-bit, that's a much smaller keyspace to 
> > brute-force attack than a 1024-bit key.
> >
> >    (do the math)
>
> You add a qualifier here -- "brute force attack" -- that makes your statement
> technichally correct, but misleading.
>
> You generally see keyspaces of 1024 bits (etc) in public key cryptosystems
> (RSA/PGP). You see 128-bit keysizes on traditional cryptosystems, like RC2,
> RC4, IDEA (the -real- encryption in PGP), etc.
>
> The problem here is that the best way to break a public-key cryptosystem
> is _not_ by brute force. RSA gets it's strength from the fact that it's very
> hard to factor a large number (1024 bits, for example) made up of two
> multiplied large primes, into it's individual primes. To break RSA, you
> 'simply' have to factor the key, which is orders of magnitude faster than
> a brute force attack on the system.
>
> Large key sizes are required for public-key cryptosystems, because HUGE
> advances are being made in number factoring. 1024-bit keys are still out
> of reach, but for how long?
>
> In the case of RC2 and RC4, the best (known -- Important word here) attack
> is a brute force attack on the key -- something that is, for the moment,
> prohibative.  Giveen huge advances in current technology, it'd still
> take YEARS to crack -one- key.

RC2 and RC4 are both public key systems -- then why wouldn't factoring 
the key prove equally as (greatly more) effective as with attacks on 
RSA/PGP. 
         __pardon_my_misunderstanding__but__?

 
> Anyhow, bottom line is that saying "RSA with a 1024 bit key is more secure
> than RC4 with a 128 bit key" is a bit silly -- You're comparing apples to 
> oranges. Nobody's going to brute-force attack RSA, since there are much better
> ways to crack the system.
>
>                                                                       -WW

Erik
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